Union of skilful means and wisdom
In order to achieve the resultant state, that is, the union of the two enlightened bodies, it is essential to engage in a path characterized by the union of method [aka skilful means] and wisdom. This fact is accepted by all Mahayana schools. However, the union of wisdom and method as explained in the sutra system is not complete. In the context of sutra, wisdom refers to the wisdom realizing emptiness, and method refers to the practice of the six perfections. The union of the two, therefore, is understood only in terms of the conjunction of two distinct factors that complement each other—that is to say, the wisdom realizing emptiness is complemented by the method aspects of the path, such as the practice of bodhichitta, compassion, and so forth. Similarly, the practice of bodhichitta and its related method aspects are complemented and supported by the wisdom factor—the realization of emptiness. In other words, in the sutra system, it is not possible to have both factors of the path—wisdom and method—complete within a single entity of consciousness.
Although it is true that when practising the union of wisdom and method in the sutra system, wisdom is not isolated from method, and method is not isolated from wisdom; still, there is no full merging of the two. Therefore, sutra practice cannot serve as the ultimate cause, or path, of actualizing the resultant state of buddhahood, in which there is a complete unity of the two enlightened bodies: Form Body and Truth Body.
The question then follows, What form of practice, or path, combines method and wisdom in an inseparable unity? The answer is the tantric practice of deity yoga. In deity yoga, a single moment of consciousness apprehends the divine form of a deity while, at the same time, being clearly aware of its empty nature. So in this case, both meditation on the deity and an apprehension of emptiness coexist in complete form within a single cognitive moment. Such a moment of consciousness, therefore, contains both of the two factors of method and wisdom. It is known as “the Vajrasattva yoga of the indivisible union of method and wisdom.” 
- *The Dalai Lama, The World of Tibetan Buddhism, published by Wisdom Publications